Connecting a Memory Card to Arduino:

7.6V - External 7.6V power (only required for 3rd party cards and knockoffs)

GND - GND Pin on Arduino

3.6V - 5V pin with a voltage divider to 3.6V

ATT - Pin 10 on Arduino

CLK - Pin 13 on Arduino

ACK - Pin 2 on Arduino

Reading save from a PC:

To read saves from the Memory Card to your PC use MemcardRex if you are using Windows.
For other operating systems use a provided Python script.
If you are writing your own application the protocol is as follows:

Interfacing with MemCARDuino:

Communication is done at 38400 bps.
To check if the MemCARDuino is connected to the selected COM port send GETID command.
Device should respond with IDENTIFIER.
Optionally you can send a GETVER to get the version of the firmware.

To Read a 128byte frame send a MCR command with MSB byte and LSB byte of the frame you want to read.
MemCARDduino will respond with 128 byte frame data, [MSB xor LSB xor Data] and Memory Card status byte.

This requires a serial port (/dev/ttyACM0 for Arduino uno's, /dev/ttyUSBX for others, COMX for Windows, and various for macOS) it also requires a specific output file.
Changing the baudrate isn't recommended, but is available anyway (it does mean changing the Arduino code manually...)